This invention relates in general to tone burst generators, and more particularly, to such generators used in telephony suitable for periodic pulse metering.
In some telephone line circuits, such as those used at public telephones, it is desirable to include tone burst generators. The generators transmit tone bursts to telephone station equipment for the purpose of incrementing call cost meters so that coin collection may be controlled at public or other telephones where toll is charged based on the time duration for calls. A key consideration in the design of such generators is to avoid audible noise caused by the tone bursts. For each telephone line circuit that is periodic pulse metered, a tone burst generator is required. It is, therefore, important to reduce the costs of such generators.
Tone burst generators have been proposed using inductors or 4-quadrant linear multiplier circuits which are expensive. Another tone burst generator is proposed in U.S. Pat. No. 5,373,552. While the above tone burst generators are useful for some applications, they are not entirely satisfactory. It is, therefore, desirable to provide an improved tone burst generator with superior characteristics.
One aspect of the invention is directed towards a tone burst generator comprising means for generating a continuous tone signal and a circuit converting said continuous tone signal into a tone burst signal having predetermined transition times, said circuit including a differential pair of a first and a second transistor, a common current path coupling the continuous tone signal to the differential pair and means for turning on and off one of the transistors to provide a tone burst signal having said predetermined transitions.
Another aspect of the invention is directed towards an apparatus for transmission of telephone signals comprising a transmission circuit including at least one first connection transmitting the telephone signals from a telephone company station to a subscriber location and at least one second connection receiving telephone signals from the subscriber location and transmitting such signals to the telephone company station and at least one generator supplying to said at least one first connection a tone burst signal having predetermined transitions. The at least one generator comprises means for generating a continuous tone signal; a differential pair of a first and a second transistor; a common current path coupling the continuous tone signal to the differential pair and means for turning on and off one of the transistors to provide a tone burst signal having predetermined transitions to provide to said at least one first connection a tone burst signal having said predetermined transitions.